This invention relates generally to the encoding, transmission, and decoding of digital information in the presence of noise on the transmission channel. More specifically, this invention relates to a unique system and method of decoding digital system information utilizing a sequence of correlation synchronization words. The coding and decoding techniques are based upon a new coding structure developed herein. Reference is made to copending application, Ser. No. 776460, filed on the same date as the present application and containing related subject matter.
Mobile radiotelephone service has been in use for some time and tranditionally has been characterized by a central site transmitting with high power to a limited number of mobile units in a large geographic area. Mobile transmissions, due to their lower power, are received by a network of receivers located remote from the central site and returned to the central site. Due to the limited number of radio channels available, a maximum number of conversations for an entire city would equal the few channels available. Consequently, mobile telephone users discovered that radiotelephone was different than landline telephone due to the often busy conditions of the channels.
To resolve this difficulty, cellular systems were developed to reuse radio channels in a given geographic area. A cellular system characteristically has the coverage area divided into contiguous smaller coverage areas (cells) using low power transmitters and receivers at the central site. One cellular system is further described in U.S. Pat. No. 3,906,166 assigned to the assignee of the present invention. The limited coverage area enables the channel frequencies used in one cell to be reused in another cell geographically separated according to a predetermined plan. One such plan is disclosed in U.S. Pat. No. 4,128,740, assigned to the assignee of the present invention. Thus, a large number of channels can be made available in a metropolitan area and the service can appear to be identical to a standard telephone.
The cell system typically utilizes one channel in each cell to receive requests for service (on a "reverse set-up" frequency) from mobile subscriber units, to call mobile subscriber units (on a "forward set-up frequency") and to instruct mobile subscriber units to tune to a frequency pair where a conversation may take place (a "voice" channel). The one "set-up" channel in each cell is continuously assigned the task of receiving and transmitting data and is the channel to which the subscriber unit tunes when not in a conversational state.
Since the cells may be of relatively small size, the likelihood of a mobile or portable subscriber unit traveling out of one cell and into another is high. To maintain communications, the subscriber unit is "handed-off" between one cell and another. The cellular systems in use track the unit and decide when a handoff is necessary to maintain quality communications. The subscriber unit is commanded, via a high speed data message interrupting the audio communications on the voice channel, to retune the transceiver to another frequency which is available in a new cell. This handoff requires a relatively short period of time and the user is generally unaware of the occurrence.
Since cellular telephone systems provide performance characteristic of the land line telephone system and interconnect with it, subscribers expect land telephone system features from the cellular telephone system. One such feature is the transmission of data from one location to another. Many telephone subscribers connect data communications devices, such as a personal computer, to the telephone system via a modem. Modems are familiar to those skilled in the art and fundamentally operate by converting data "1" and ".0." levels to distinct tones or to particular tone waveform phase relationships which can be transmitted by the land telephone network.
It would be natural to connect a computing device via a modem to a radiotelephone subscriber unit for communication with another data generating device via the land telephone network. In fact, this has been done and produced unsatisfactory results. Rapid multipath fading, commonly experienced in high frequency cellular radiotelephone communications, causes gaps and significant phase changes in modem-generated tones such that data carried by the radio channel becomes garbled or missing. Furthermore, a handoff between cells, which to a human engaged in conversation is virtually unnoticeable, becomes a formidable obstacle for the communication of data generated by a data generating device.
This limitation has been resolved by converting the input data to a data format compatible with radio channel transmission and reconverting the radio channel data format back to the original format upon reception. The data transmission is halted prior to a handoff and is resumed after the handoff is completed. A detailed description of the above process may be found in U.S. patent application No. 630,481 filed on July 13, 1984 in behalf of Labedz, et al. and assigned to the assignee of the present invention.
A second feature of the landline telephone system which subscribers to a cellular radiotelephone system wish to have is that of relative security of their conversations. Digital scrambling techniques for secure radio communication channels has been disclosed in U.S. Pat. Nos. 4,167,700; 4,434,323; and 4,440,976 each assigned to the assignee of the present invention. These inventions, however, do not address the aforementioned handoff requirement of cellular systems and do not provide the necessary transponding of a supervisory signal to maintain communications on an assigned channel.
Cellular systems, to reduce interference between cochannel users, employs several distinguishable signals (SAT signals) to identify cellular fixed sites. Each site is assigned one signal which is transmitted with each transmission from fixed site to subscriber unit. The subscriber unit, in turn, transponds the same signal to the fixed site and the radio channel connection is maintained. If anomalous propagation enables cochannel transmission to or from one cell to be received in another cell, the lack of a properly transponded signal will cause the radio channel to be deallocated from the interfering subscriber unit. These distinguishable SAT signals typically are unique tone frequencies for analog transmissions and unique bit sequences for digital transmissions.
The use of a digital frame synchronizing bit sequence for the system tasks of synchronizing, supervisory signal (SAT), and system control message transmission has been disclosed in the previously mentioned copending U.S. patent application Ser. No. 771,460. Conventional coding and decoding methods do not take full advantage of the latent error detection and correction power of a coding technique employing a sequence of multiple bit synchronization words to carry information. Error detection and correction, as previously practiced, relies upon redundancy of information, correlation techniques, or parity to correct one or more bits in an information bit string. Each bit in the information string is either correct or in error. The present invention utilizes a third possible state in the information bit string - a "missed" bit. Each "bit" in the system control message of the present invention is derived from a synchronization bit sequence word, itself consisting of a plurality of bits.